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Determination of residual concentration of ionic liquids with different anions and alkyl-chain lengths in water and soil samples Tongtong Zhou, Jinhua Wang, Cheng Zhang, Jun Zhang, Lusheng Zhu, Zhongkun Du, and Jun Wang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b02693 • Publication Date (Web): 02 Sep 2017 Downloaded from http://pubs.acs.org on September 3, 2017

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Analytical Chemistry

Determination of residual concentration of ionic liquids with different anions and alkyl-chain lengths in water and soil samples

Tongtong Zhou#, Jinhua Wang#, Cheng Zhang, Jun Zhang, Lusheng Zhu*, Zhongkun Du, Jun Wang College of Resources and Environment, Key Lab of Agricultural Environment in Universities of Shandong, Shandong Agricultural University, Taian 271018, People Republic of China

Email: Tongtong Zhou: [email protected] Jinhua Wang: [email protected] Cheng Zhang: [email protected] Jun Zhang: [email protected] Lusheng Zhu:[email protected] Zhongkun Du: [email protected] Jun Wang: [email protected]

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Tongtong Zhou and Jinhua Wang contributed equally to this work.

*Corresponding Author: Lusheng Zhu* College of Resources and Environment, Shandong Agricultural University, 61 Daizong Road, Taian 271018, China Tel: +86 538 8249789 Fax: +86 538 8242549 Email: [email protected]

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ABSTRACT Considering the wide synthesis and application of ionic liquids (ILs), the toxicity of ILs has recently gained growing attention. However, few studies focused on IL determination methods in environmental samples. In the present study, we implemented the determination methods for the 12 ILs with different chemical structures using high performance liquid chromatography (HPLC) and ultraviolet (UV) spectrophotometry. The optimum conditions for extraction of ILs from soil samples were also obtained by single-factor experiments and response surface methodology (RSM). The instrument detection limits (IDLs) reached 10-10 g. Compared to the use of UV, HPLC had the standard curve with stronger correlation (R2 ≥ 0.999) and lower detection limit. We therefore used HPLC to detect the contents of ILs in water and soil samples. A standard adding method was used for the reliability test of the above methods. The average recovery in water samples was 90.46%-108.83% and the coefficient of variation (CV) was 0.51%-9.07%. The method detection limits (MDLs) were below 0.1 mg/L. The optimized IL extraction conditions in soil samples were as follows: The ratio of methanol and saturated ammonium chloride was 90:10, the ultrasonic time was 50 min and power was 350 W. The average recovery in soil samples was 70.39%-85.30% and the CV was 0.50%-9.99%. The MDLs were below 1 mg/kg. These results using the aforementioned methods met the standards of residue analysis. The present study can provide scientific analysis methods and a basis for evaluation of the study of IL residues in environmental samples.

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KEYWORD ILs; environmental sample; green solvent; detection; high performance liquid chromatography (HPLC); ultraviolet spectrophotometry

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INTRODUCTION Ionic liquids (ILs) are new type of room temperature melting salt that are entirely composed of ions.1,2 Because their structures can be easily tuned, different ILs can be synthetized by varied combinations of different cations and anions.3–5 Their unique structures impart many unique physical and chemical properties, such as a wide range of dissolution, non-volatility, good conductivity, low vapor pressure and good thermal stability.6–8 Therefore, ILs have been widely used in various applications, such as organic synthesis, electrochemistry, catalysis and separation processes.9–12 However, the increased current and future use of ILs may bring about their release into water and soil environment through accidental leaks or irrigation or via effluents.13–15 Although ILs are deemed as “green” solvents, a some of their properties, such as their light degradation resistance and water stability, increase their risk to the environment.16–18 A study in bacteria found that IL toxicity was associated with alkyl chain length,10,19 and the effects of anion toxicity have been easily obtained.20–23 A great number of studies have reported the influence of ILs on water and soil environments at different biological levels, such as microorganisms, algae, earthworms, mice and phylogenetically higher animals and plants.24–28 All physical and chemical studies of environmental biology require efficient and accurate analytical techniques. These methods must be suitable for not only different environmental samples but also the extremely low concentrations that exist in environmental systems.29 Because of their above-mentioned physical and chemical properties mentioned, the determination of IL contents could not use gas chromatograph (GC). And through reading relevant literature, it was found that high performance liquid chromatography (HPLC) was 4


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widely used.30,31 Although some analysis and extract methods have been established from the present reports,32,33 more reliable and effective methods should be studies for more different ILs with the research advancing. Consequently a study on the determination and extraction methods of various ILs from water and soil samples is of great significance. On the basis of these considerations, we focus our concern on the determination of IL contents in water and soil environmental samples. Here, the frequently used ILs 1-alkyl-3-methylimidazolium chloride ([Cnmim]Cl, n = 4, 6, 8, 10), 1-alkyl-3-methylimidazolium bromide ([Cnmim]Br, n = 4, 6, 8, 10) and 1-alkyl-3-methylimidazolium tetrafluoroborate ([Cnmim]BF4, n = 4, 6, 8, 10), which have different anions and alkyl-chain lengths, were chosen for determination. The ultraviolet spectroscopy and HPLC methods were examined. Furthermore, the conditions for extraction of ILs from soil samples were optimized using response surface methodology (RSM) with a Box-Behnken design (BBD). Response surface methodology is a statistical means for the design of experiments, the evaluation of multiple factor effects, the construction of models, and the investigation of optimal conditions.33 The best conditions were successively applied to extract ILs from soil samples.

MATERIALS AND METHODS Materials. The ILs [C4mim]Cl (CAS NO. 79917-90-1), [C4mim]Br (CAS NO. 85100-77-2), [C4mim]BF4 (CAS NO. 174501-65-6), [C6mim]Cl (CAS NO. 171058-17-6), [C6mim]Br (CAS NO. 85100-78-3), [C6mim]BF4 (CAS NO. 244193-50-8), [C8mim]Cl (CAS NO. 64697-40-1), [C8mim]Br (CAS NO. 61545-99-1), [C8mim]BF4 (CAS NO. 244193-52-0), 5



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[C10mim]Cl (CAS NO. 171058-18-7), [C10mim]Br (CAS NO. 188589-32-4) and [C10mim]BF4 (CAS NO. 244193-56-4) had a 99% purity and were obtained from Chengjie Chemical Co. Ltd. (Shanghai, China). All stock solutions of each investigated IL in the present study were prepared at a concentration of 1000 mg/L with deionized water and were stored at 4°C in a refrigerator. All other chemicals purchased from Beijing Chemical Co. (Beijing, China) were of analytical reagent grade except HPLC gradient grade methanol and acetonitrile which were chromatographically pure.

Instrumentation and Analytical Procedures of UV. Each ILs stock solution (5 mL) was diluted with deionized water to obtain working solutions with a concentration of 50 mg/L. The working solutions were then transferred to a 10 mm quartz cuvette. An ultraviolet-visible spectrophotometer (UV/VIS, Shimadzu, UV-2600) was used to measure the maximum absorption wavelength from 199 to 400 nm with deionized water as a reference. The UV parameters were set as follows: the scanning speed was medium speed, the sampling interval was 0.1, the automatic sampling interval was activation, the scanning mode was single-mode, and the slit width was 2.0 nm. Next we established the calibration curves. Working standard solutions were prepared at concentrations of 5, 10, 15, 20, 30, 35, 40, 45, 50 mg/L by diluting stock solutions with deionized water. Then the absorbance of ILs [Cnmim]Cl, [Cnmim]Br and [Cnmim]BF4 were measured at the corresponding maximum absorption wavelengths, with deionized water as a reference. Finally, the calibration curve was obtained with the absorbance and the concentrations of IL as the ordinate and abscissa respectively.

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Instrumentation and Analytical Procedures of HPLC. First, working standard solutions of each IL were prepared at concentrations of 0.1, 0.5, 1.5, 10, 15, 20 mg/L by diluting stock solutions with deionized water. Then these working standard solutions were passed through a 0.45 µm syringe filter and analyzed by high-performance liquid chromatography (HPLC, Agilent 1100, USA), which was equipped with a C18 column (Eclipse XDB-C18, 4.6 × 250 nm, 5 µm). The column temperature was maintained at 30 ◦C, the wavelength of determination was 212 nm, the flow rate was 0.8 mL/min and the injection volume was 10 µL. In the case of [C4mim]R (R = Cl-, Br-, BF4-) the mobile phase was a mixture of methanol (30%, v/v) and 25 mM of phosphate buffer (KH2PO4/H3PO4) in 0.5% triethylamine (pH 3.0). For the analysis of [C6mim]R (R = Cl-, Br-, BF4-),[C8mim]R (R = Cl-, Br-, BF4-) and [C10mim]R (R = Cl-, Br-, BF4-), the mobile phase was the mixture of acetonitrile (30%, 35%, 40%, v/v) and 25 mM of phosphate buffer (KH2PO4/H3PO4) in 0.5% triethylamine (pH 3.0). Finally, standard curves were established with concentration as the abscissa and peak area from HPLC as the ordinate. The instrument detection limits (IDLs) were recorded when the signal to noise ratio (S/N) was greater than or equal to 3.

The Analytical Methods to Determine the Contents of ILs in Water Analytical procedures: Unknown water sample were transferred to a 10 mL centrifuge tube and centrifuged at 4000 rpm for 10 min (Centrifuge 5804, Eppendorf, Germany). The resultant supernatants were passed through a 0.45 µm syringe filter and analyzed via HPLC. After detection by HPLC, we obtained the peak area and then calculated the IL concentration of samples according to the formal standard curves. 7



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Reliability experiment: We adopted a standard adding method to verify the reliability of the methods. The reliability and precision of the methods were calculated by the recovery and the coefficient of variation of the results. The water used in this part was tap water at a temperature of 26 ± 1°C, pH 7.4-8.1, and did not contain any ionic liquids. The standard solutions of each tested ionic liquids were added to the water to make the concentration of the water samples of 0.1, 1, 10, and 100 mg/L. Then, the samples were analyzed by the above procedures. And the samples at the concentrations of 100 mg/L were determined after being diluted 10 times. Each group underwent three parallel experiments, and the recovery and coefficient of variation were calculated.

Optimization of Methods to Extract ILs from Soil Soil samples: The soil used in the present study was collected from the test field of Shandong Agricultural University (Taian, China), and the sample depth was 2-20 cm. The organic matter content was 17.6 ± 1.1 mg/kg. The available potassium content was 125.7 ± 7.4 mg/kg. The organic nitrogen content was 132.3 ± 9.6 mg/kg. The available phosphorus content was 18.4 ± 1.6 mg/kg. The pH of the soil was 7.6 ± 0.4. The maximum field capacity was 18.5 ± 1.4%. The soil was air-dried and passed through a 2 mm sieve. The moisture in the dried-soil was approximately 4%.

Extraction procedures: Each poisonous soil sample (2 g) was transferred to a test tube (18 × 18 mm) and then kept until addition of the extraction solution (10 mL). The extraction 8


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solution was prepared by methanol and saturated ammonium chloride at the optimal ratio.32 Ethylenediaminetetraacetic acid disodium salt (EDTA-2Na, 1 g/L) was added to the extraction solution to bind the metal ions in the soil sample. After a 30 second turbine mix, the sample was extracted at a certain extract power and time. The ratio of methanol in the extraction solution, as well as the extraction time and power, were determined by a single-factor experiment and RSM. A turbine then mixed and shook the solution for 1 h (30°C, 160 rpm). The resultant supernatants were then transferred to a 10 mL centrifuge tube and centrifuged at 4000 rpm for 10 min. Then, the supernatants were passed through a 0.45 µm syringe filter and analyzed via HPLC.

Single factor experiment for the influencing factors for the IL extraction rate: ILs [C6mim]Cl, [C8mim]Cl, [C10mim]Cl, [C10mim]Br and [C10mim]BF4 were added to the soil samples (2 g) to reach a concentration of 100 mg/kg as determined by the methods discussed above. The influence of different proportion of methanol was determined under a constant ultrasonic time (1 h) and power (300 W). The proportion of methanol was designed at 0, 10, 30, 50, 70, 90, 95 and 100%. Each group underwent three parallel experiments. The influence of different ultrasonic times was determined under a constant extraction solution (the proportion of methanol was 90%) and ultrasonic power (300 W). The ultrasonic time was designed at 10, 30, 50, 70 and 90 min. Each group underwent three parallel experiments. The influence of different ultrasonic power was determined under a constant extraction solution (the proportion of methanol was 90%) and ultrasonic time (1 h). The ultrasonic 9



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power was designed at 200, 250, 300, 350, 400 and 450 w. Each group underwent three parallel experiments.

Response surface optimization: The BBD was established with the methanol ratio (A), extraction time (B) and power (C) as independent variable and the extraction rate of ILs as the response after measuring the preliminary extraction conditions through single-factor experiments. These factors and levels are listed in Table S1. The connection between the experimental and response levels was depicted using surface and contour plots conclude from the fitted polynomial equation.34 The effect and regression coefficients of the individual linear, quadratic, and interaction terms were measured and analysis of variance (ANOVA) tables were created.35

Reliability experiment: We adopted a standard adding method to verify the reliability of the previously described methods. The reliability and precision of the methods were calculated by the recovery and coefficient of variation of the results. We chose nine frequently used ILs [C6mim]R, [C8mim]R and [C10mim]R, and the IL standard solutions were added to the soil to make the concentration of 1, 10, and 100 mg/kg. Then, the samples were analyzed by the above procedures. Each group underwent three parallel experiments, and the recovery and coefficient of variation were calculated.

Statistical Analysis. The Statistical Package for Social Sciences (SPSS, Standard Version 21.0, SPSS Inc., USA) software program and Microsoft Excel (Edition 2010, Microsoft Corp., 10


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USA) were adopted for data analysis and all data are expressed as the means ± standard deviation (SD). The least significant difference (LSD) test was used to analyze the significant differences between the treatments and the control (p < 0.5).

RESULTS AND DISCUSSION Results for UV. Wilkes believed that ILs had no harmful for the experimenters, provided the skin of the experimenters did not directly touch the ILs.36 Through determination, the maximum absorption of the ILs [Cnmim]Cl and [Cnmim]BF4 was 212 nm, and that for the ILs [Cnmim]Br was approximately 200 nm. Table 1 illustrates the results of UV, which lists the correlation coefficients (R2) and regression curve parameters for the working standard solutions. The calibration curves displays linearity over concentrations from 0 to 50 mg/L, with an R2 from 0.9991 to 0.9998 for the working standard solutions of ILs [Cnmim]Cl and from 0.9987 to 0.9996 for the working standard solutions of ILs [Cnmim]BF4. For ILs [Cnmim]Br, the calibration curves were linear over concentrations from 0 to 50 mg/L, with an R2 from 0.9994 to 0.9996 for [C8mim]Br and [C10mim]Br. For ILs [C4mim]Br and [C6mim]Br, the calibration curves were linear over the concentration ranges of 0 to 25 mg/L and 0 to 35 mg/L, and the R2 values were 0.9974 and 0.9928, respectively. Put Table 1 here. The UV method is based on the Lambert-Beer theory, which is approximate for solutions at low concentrations. The absorbance and concentration had a linear relation at an absorbance between 0.2 and 0.8. If the absorbance was too big or small, the absorbance was not proportion to the concentration. From the calibration curves, we obtained the optimum 11



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experimental concentrations for all ILs, which are shown Table 1. Because the organic cations in ILs have a characteristic absorption in the ultraviolet region, the UV method could be used. As a result, the impurities in water samples, which had characteristic absorptions, impacted the accuracy and sensitivity of the results. Consequently, the UV method was easy to operate but might be applicable to the analysis of the water samples which were clear and had less impurity.

Results of HPLC. Table 2 shows the correlation coefficients (R2) and regression curve parameters for HPLC. All calibration curves demonstrated linearity over the concentration range of 0 to 20 mg/L, with R2 values from 0.9991 to 1 for all tested ILs. The instrument detection limits (IDLs) for HPLC were below 10-10 g. In the exiting studies, the ILs content determination had been reported.37–39 Eleven 1-alkyl-3-methyl imidazolium ionic liquid (IL) salts were analyzed in reversed phase mode with a Kromasil C18 column. The mobile phases were water-rich acetonitrile solutions (water content ≥ 70%, v/v) without any added salts.30 The separation of selected 1-alkyl- and 1-aryl-3-methylimidazolium-based room temperature ionic liquid cations has been performed using reversed-phase high-performance liquid chromatography with electrospray ionization mass detection.31 The conditions were confirmed against such previous references. In addition, we carried out numerous experiments to confirm the final conditions. From the results, the conditions we built were appropriate for determination. Collectively, we found that HPLC was more practical and had a higher accuracy than UV. Therefore, we decided to use HPLC to analyzed ILs in water and soil samples. 12


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Put Table 2 here.

Results of Determination of IL Contents in Water. The concentrations of ILs in water samples were determined by HPLC. The repeatability of the methods was determined by the recovery rates with the correlation of variation (CV) listed in Table 3. From Table 3, we found that the recovery rates of all ILs were within 90.46% to 108.83% and the coefficient of variation was within 0.51% to 9.07%. The method detection limits (MDLs) were below 0.1 mg/L. The method satisfied the requirements of residue analysis for accuracy and precision. Through analyzing the result, we found that the CV at high concentrations was less than that at low concentrations. The possible reason is the smaller peak area for ILs at low concentrations, which could cause the integration error of the peak area to be greater. Both HPLC and UV/VIS could determine the IL contents in water samples, but the UV method was greatly affected by the presence of other ions and compounds in the solution. Yu et al. measured the contents of seven types of imidazolium-based ionic liquids in water using UV.40 Furthermore, Lamouroux et al. used HPLC and hydrophilic interaction liquid chromatography method to determinate the five types of imidazolium-based ILs.41 However, the IL contents of environment samples were not detected. Our results thus were useful as a supplement to the abovementioned results. Put Table 3 here.

Optimization Result for IL Extraction Conditions using RSM Results of single factor experiments on IL extraction rate: Figure 1 shows the optimum 13



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conditions for the extraction of ILs [C6mim]Cl, [C8mim]Cl, [C10mim]Cl, [C10mim]Br and [C10mim]BF4: the ratio of methanol and saturated ammonium chloride was 90:10, the ultrasonic time was 50 min and the ultrasonic power was 350 W. Effects of different factors on IL extraction rates showed the same trends, with an increase in the methanol ratio, ultrasonic time and ultrasonic power, the IL extraction rate increased initially and then decreased. The extraction rate decreased with increasing ultrasonic time after 50 min. This could be due to the fact that over time, ILs absorb in soil, which can lead to a decline in the extraction rate. In regard to the changed trends of the methanol ratio and ultrasonic power, further studies are needed to elucidate the mechanism. Put Figure 1 here

Results of RSM: The methanol ratio, ultrasonic time and power were optimized to obtain optimal process conditions and establish regression model through response surface methodology, based on single-factor tests. Levels, factors, and responses (extraction rate) for the five ILs for each test included in Table S2 to Table S6. The relationship between the extraction rate and individual parameters of different ILs was calculated by the second-order polynomial equations using the multiple regression analysis applied to the experimental data, which had the following definitions: [C6mim]Cl extraction rate (%) = 86.23 – 3.90 × A + 0.68 × B + 0.33 × C + 0.05 × A × B – 0.32 × A × C – 0.77 × B × C – 5.25 × A2 – 0.50 × B2 – 0.54 × C2, [C8mim]Cl extraction rate (%) = 82.8 – 1.9 × A + 0.34 × B + 0.24 × C + 0.1× A × B – 0.3 × A × C – 0.96 × B × C – 3.26 × A2 – 0.35 × B2 – 0.89 × C2, [C10mim]Cl extraction rate (%) = 81.33 – 2.13 × A + 0.05 × B + 14


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0.18 × C – 0.07 × A × B – 0.37 × A × C – 0.56 × B × C – 2.97 × A2 – 1.06 × B2 – 0.42 × C2, [C10mim]Br extraction rate (%) = 80.568 – 0.78 × A + 0.78 × B + 0.56 × C – 0.21 × A × B – 0.24 × A × C – 0.06 × B × C – 2.78 × A2 – 1.34 × B2 – 1.42 × C2, [C10mim]BF4 extraction rate (%) = 81.22 – 2.18 × A + 0.35 × B + 0.10 × C – 0.02 × A × B + 0.07 × A × C + 0.003 × B × C – 2.76 × A2 – 0.72 × B2 – 0.65 × C2. (A is the methanol ratio, B is the ultrasonic time, C is the ultrasonic power). The results of analysis of variance for ILs [C6mim]Cl, [C8mim]Cl, [C10mim]Cl, [C10mim]Cl, [C10mim]Br and [C10mim]BF4 are performed using the Design-Expert 8.05b analysis of variance and are listed from Table S7 to Table S11. The results show that every response surface model we established was highly significant with p < 0.01, R2 ≥ 0.9687, and adjusted R2 ≥ 0.9285. Between the experimental data and fitted model, good agreements were found, which suggested that the model has a high prospect for predicting the IL extraction rate. The lack of fit was not significantly associated with pure error according to the lack-of-fit p value between 0.1266 and 0.2519. Furthermore, different factors had different degree effects on the IL extraction rate: methanol ratio > ultrasonic time > ultrasonic power. Figure 2 intuitively reflects the effects of various factors on the response values. As shown in Figure 2, the methanol ratio was an important factor that could affect the IL extraction rate, which could be seen from the steep response surfaces. For ultrasonic time and ultrasonic power, the response surfaces were comparatively smooth. The results were consistent with those of the analysis of variance. According to the above results and considering the conveniences of actual practice, we amended the conditions as follows: a ratio of methanol and saturated ammonium chloride of 90:10, an ultrasonic time of 50 min and an 15



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ultrasonic power of 350 W. Response surface methodology is a statistical method for solving issues of nonlinear data processing and has widely been used for the optimization of a large number of extraction technologies. Kalil et al. used factorial and RSM in combination with modeling and simulation to design and optimize an industrial bioprocess.42 Chen et al. optimized a method for extracting 16 priority polycyclic aromatic hydrocarbons from biochar-based fertilizers.35 Thus, to obtain the optimal extraction methods for ILs in soil we chose RSM. Put Figure 2 here.

Results of the reliability experiment: Table 4 shows that the recovery rates of the ILs [C6mim]R, [C8mim]R and [C10mim]R in soil samples were within 70.39% to 85.30% and the coefficient of variation was within 0.50% to 9.99%. The MDLs were below 0.1 mg/L. The method satisfies the requirements of residue analysis for accuracy and precision. Put Table 4 here. From the results, we found that different alkyl chain lengths and concentrations had an impact on the extraction rate but different anion had little or no effect. The results showed that the extraction rate decreased with increasing alkyl chain lengths at a given concentration. Approximately the following trend was found: [C6mim]R > [C8mim]R > [C10mim]R (R = Cl-, Br-, BF4 -). The possible reason for this appearance is that the absorption ability of ILs increased with alkyl chain lengths, which makes them difficult to extract. In addition, the extraction rate decreased with decreasing concentrations of ILs in soil samples. The probable cause was the fact that less IL fully came into contact with the soil, which was in favor of 16


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absorption. Nichthauser et al. extracted 1-hexyl-3-methylimidazolium from four kinds of soil and the extraction rate was approximately 28%-40%.32 Our optimized method presented a higher extraction rate. The reason may be that the optimization of the extraction conditions through the single-factor experiments and response surface methodology, as well as the different physical and chemical properties of the soil samples.

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SUPPORTING INFORMATION Detailed information on the calculation procedure adopted for optimizing the extraction conditions of ILs from soil samples using RSM. Tables show the experimental design for response surface analysis and analysis of variance of [C6mim]Cl, [C8mim]Cl, [C10mim]Cl, [C10mim]Br and [C10mim]BF4.

CONFLICT STATEMENTS The authors declare no competing financial interest.

ACKNOWLEDGEMENT This work was supported by the National Key Research and Development Plan (No. 2016YFD0800202, 2017YFD0200307 and 2016YFD0201203); the National Natural Science Foundation of China (No. 41771282, 41671320); the Natural Science Foundation of Shandong Province, China (ZR2017MD005) and the Special Funds of Taishan Scholar of Shandong Province, China.

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Laboratory 2008, 25, 114–117. (41) Lamouroux, C.; Foglia, G.; Rouzo, G. L. J. Chromatogr. A. 2011, 1218, 3022–3028. (42) Kalil, S. J.; Maugeri, F.; Rodrigues, M. I. Process Biochem. 2000, 35, 539–550.

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FIGURE CAPTIONS Figure 1. Effect of different proportion of methanol (A), ultrasonic time (B) and power (C) on extraction rate of ILs [C6mim]Cl, [C8mim]Cl, [C10mim]Cl, [C10mim]Br and C10mim]BF4 from soil samples. Each group underwent three parallel experiments.

Figure 2. Response surfaces of the pairwise interactive effects of three extraction conditions on extraction rate of ionic liquid [C6mim]Cl (A), [C8mim]Cl (B), [C10mim]Cl (C), [C10mim]Br (D) and [C10mim]BF4 (E) from soil samples. From left to right the response surfaces were time versus methanol ratio, power versus methanol ratio and power versus time.

23



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FIGURES

A

B

24


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C

Figure 1. Effect of different proportions of methanol (A), ultrasonic time (B) and ultrasonic time (C) on extraction rate of ILs [C6mim]Cl, [C8mim]Cl, [C10mim]Cl, [C10mim]Br and [C10mim]BF4 from soil samples. Each group underwent three parallel experiments.

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

A

B

C

D

E

Figure 2. Response surfaces of the pairwise interactive effects of three extraction conditions on extraction rate of ionic liquid [C6mim]Cl (A), [C8mim]Cl (B), [C10mim]Cl (C), [C10mim]Br (D), [C10mim]BF4 (E) from soil samples. And from left to right the response surfaces were time versus methanol ratio, power versus methanol ratio and power versus time.

26


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TABLES Table 1. Regression equations and the best range of measurement of twelve ILs by using UV Linear

Optimum

correlation

determination

coefficients

range (mg/L)

Linear range ILs

Regression equation (mg/L)

[C4mim]Cl

0-50

y = 0.0295x – 0.0074

0.9991

7-27

[C6mim]Cl

0-50

y = 0.0205x – 0.0474

0.9992

12-24

[C8mim]Cl

0-50

y = 0.0204x – 0.0269

0.9998

11-40

[C10mim]Cl

0-50

y = 0.0151x – 0.0015

0.9993

13-50

[C4mim]Br

0-25

y = 0.0776x + 0.0165

0.9974

2-10

[C6mim]Br

0-35

y = 0.0594x – 0.0006

0.9928

3-13

[C8mim]Br

0-50

y = 0.0422x – 0.0225

0.9994

5-19

[C10mim]Br

0-50

y = 0.0413x – 0.0121

0.9996

5-19

[C4mim]BF4

0-50

y = 0.0209x – 0.0068

0.9991

10-39

[C6mim]BF4

0-50

y = 0.0367x – 0.0435

0.9996

7-23

[C8mim]BF4

0-50

y = 0.0111x – 0.0290

0.9987

20-50

[C10mim]BF4

0-50

y = 0.0193x – 0.0192

0.9993

11-42

27



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Page 28 of 33

Table 2. Regression equations and limits detections of twelve ILs by using HPLC

Linear range Mobile phase conditions

ILs (mg/L)

Linear

Instrument

correlation

detection

coefficients

limit (g)

Regression equation

Methanol : phosphate buffer = 30 : 70

[C4mim]Cl

0-20

y = 20.005x – 0.2179

1

6×10-10


[C4mim]Br

0-20

y = 15.416x + 0.9654

0.9999

9×10-10


[C4mim]BF4

0-20

y = 16.961x – 0.5652

1

6×10-10

Acetonitrile : phosphate buffer = 30 : 70

[C6mim]Cl

0-20

y = 14.195x + 0.7451

0.9995

3×10-10


[C6mim]Br

0-20

y = 11.907x + 0.8375

0.9996

5×10-10


[C6mim]BF4

0-20

y = 11x – 1.2508

0.9994

3×10-10


[C8mim]Cl

0-20

y = 12.522x – 0.2946

0.9999

5×10-10


[C8mim]Br

0-20

y = 10.734x – 0.0641

0.9991

7.5×10-10


[C8mim]BF4

0-20

y = 10.892x – 0.1258

0.9999

5×10-10

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[C10mim]Cl

0-20

y = 11.974x – 0.3945

0.9999

8×10-10


[C10mim]Br

0-20

y = 9.8528x – 0.1994

0.999

8×10-10


[C10mim]BF4

0-20

y = 10.292x – 0.0169

0.9993

8×10-10

29



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Page 30 of 33

Table 3. Recovery of 12 ionic liquids from water samples Recovery

Average

Coefficient

(%)

Recovery

of variation

Spiked level ILs (mg/L)

[C4mim]Cl

[C4mim]Br

[C4mim]BF4

[C6mim]Cl

[C6mim]Br

[C6mim]BF4

1

2

3

(%)

(%)

0.1

100.87

105.87

115.87

107.53

5.80

1

91.07

98.56

102.56

97.40

4.89

10

101.08

102.58

103.83

102.50

1.10

100

101.38

104.58

103.28

103.08

1.27

0.1

86.57

93.06

99.55

93.06

5.69

1

98.17

96.88

99.47

98.17

1.08

10

101.54

102.58

103.81

102.64

0.90

100

106.60

105.24

103.29

105.04

1.29

0.1

90.49

96.39

108.18

98.35

7.48

1

90.41

94.54

96.90

93.95

2.85

10

106.15

105.14

106.38

105.89

0.51

100

98.30

99.31

97.07

98.23

0.93

0.1

88.40

95.45

102.49

95.45

6.03

1

99.01

106.06

86.33

97.13

8.40

10

97.54

95.99

98.81

97.56

1.18

100

98.59

99.51

97.12

98.41

1.00

0.1

106.03

106.03

114.43

108.83

3.64

1

104.67

102.15

114.74

107.18

5.08

10

99.24

101.09

98.73

99.69

1.01

100

95.79

96.72

98.23

96.91

1.04

0.1

104.47

95.38

86.29

95.38

7.78

1

103.17

110.45

95.90

103.17

5.76

10

94.95

97.95

100.14

97.68

2.17

100

103.04

101.41

104.23

102.89

1.12

30


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[C8mim]Cl

[C8mim]Br

[C8mim]BF4

[C10mim]Cl

[C10mim]Br

[C10mim]BF4

0.1

111.37

103.39

103.39

106.05

3.55

1

98.98

107.77

106.17

104.31

3.66

10

110.12

103.49

101.10

104.90

3.64

100

103.49

97.26

100.54

100.43

2.53

0.1

96.51

105.82

87.19

96.51

7.88

1

101.88

86.04

97.22

95.05

6.99

10

100.83

95.80

99.90

98.85

2.21

100

94.97

94.50

98.69

96.05

1.95

0.1

112.54

94.18

103.36

103.36

7.25

1

102.15

111.33

97.56

103.68

5.52

10

96.88

95.97

94.59

95.81

0.98

100

90.92

94.59

94.68

93.39

1.88

0.1

116.46

108.11

99.76

108.11

6.31

1

103.51

106.85

101.01

103.79

2.31

10

109.23

101.30

101.38

103.97

3.58

100

96.20

98.79

96.70

97.23

1.15

0.1

91.41

101.55

81.26

91.41

9.07

1

88.31

96.43

89.32

91.35

3.95

10

101.19

102.21

99.36

100.92

1.17

100

105.45

99.77

100.28

101.83

2.52

0.1

100.17

90.46

80.74

90.46

8.77

1

91.63

101.35

95.52

96.17

4.15

10

105.45

101.66

103.02

103.38

1.52

100

96.22

95.06

100.30

97.19

2.31

31



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Page 32 of 33

Table 4. Recovery of nine ionic liquids from soil samples Spiked

Extraction ratio (%)

Average Coefficient of

ILs

level

extraction ratio 1

2

(mg/kg)

[C6mim]Cl

[C6mim]Br

[C6mim]BF4

[C8mim]Cl

[C8mim]Br

variation (%)

3 (%)

1

83.74

66.13

74.94

74.94

9.59

10

79.35

79.88

77.76

79.00

1.14

100

85.58

86.49

83.82

85.30

1.30

1

70.60

85.30

68.50

74.80

9.99

10

79.29

80.76

84.12

81.39

2.48

100

83.17

84.43

85.73

84.44

1.24

1

87.48

78.39

71.57

79.15

8.23

10

82.38

81.02

85.57

82.99

2.29

100

84.89

86.48

83.57

84.98

1.40

1

68.82

79.81

81.80

76.81

7.43

10

77.85

80.25

80.65

79.58

1.55

100

84.13

80.42

82.41

82.32

1.84

1

70.29

84.26

77.28

77.28

7.38

10

81.37

80.44

79.27

80.36

1.07

100

80.09

81.07

80.56

80.57

0.50

1

79.48

72.60

84.07

78.72

5.99

10

86.36

75.80

80.62

80.93

5.33

[C8mim]BF4

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[C10mim]Cl

[C10mim]Br

[C10mim]BF4

100

80.94

83.01

79.93

81.29

1.57

1

81.31

68.78

77.14

75.74

6.88

10

75.36

70.77

80.37

75.50

5.19

100

82.55

79.42

80.67

80.88

1.59

1

68.52

71.06

76.14

71.91

4.40

10

81.70

77.90

78.41

79.34

2.13

100

81.04

81.75

80.28

81.03

0.74

1

73.63

71.20

66.34

70.39

4.30

10

79.51

79.75

77.08

78.78

1.53

100

80.08

82.02

80.81

80.97

0.99

33


Determination of Residual Concentration of Ionic ... - ACS Publications

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